Quota exceed leading to wrongly calculated usage

We are facing that calculation of the individual usage of mails are wrong. It seems to happen when the quota has been exceeded. We are running 11.26.20.

When looking at the 'Disk Space usage' it shows as an example 154 MB while the mail box is reporting 504 MB.

How can this happen and how is the mail box usage calculated? We need each time to bother our supplier for having the calculation corrected. We suspect that this is a bug or a wrong implementation of the counting of mail box size.